The invention relates to a turbine engine starter-generators.
The field of application of the invention is more particularly that of starter-generators for gas-turbine propulsion aeroengines or for gas-turbine auxiliary power units (APU) mounted on board aircraft. Nevertheless, the invention is applicable to other types of turbine engine, for example industrial turbines.
Such a starter-generator (S/G) conventionally comprises a main electrical machine that forms a main electricity generator operating in synchronous mode after the associated turbine engine has started and ignited. The main electrical machine possesses a rotor inducer and stator windings that, in synchronous generator mode, deliver electrical energy in the form of alternating current (AC) to a network on board an aircraft via a power supply line having a line contactor mounted thereon. The AC delivered by the main generator is regulated by means of a generator regulation unit or generator control unit (GCU) that delivers direct current (DC) to a stator inducer of an exciter having its rotor windings connected to the rotor inducer of the main electrical machine via a rotary rectifier. The electrical energy needed for powering the inducer of the exciter may be delivered by an auxiliary electricity generator such as a permanent magnet synchronous generator, or it may be derived from the on-board electricity network of the aircraft.
The rotors of the main electrical machine, of the exciter, and of the auxiliary generator, if any, are mounted on a common shaft that is mechanically coupled to a shaft of the turbine engine, and they constitute a brushless starter-generator having two or three stages.
In order to start the turbine engine, it is well-known to operate the main electrical machine in synchronous electric motor mode by powering its stator windings with AC delivered on the power supply line via the line contact, and by powering the rotor inducer via the exciter. Since the shaft of the starter-generator is initially at rest, it is necessary to act via the GCU to inject AC into the stator inducer of the exciter in order to develop AC in its rotor windings that, after being rectified, serves to power the rotor inducer of the main electrical machine.
In order to be able to inject the AC needed for producing the torque required for starting, the GCU needs to be designed and dimensioned to be much greater than is necessary for powering the exciter with DC in generator mode.
In order to remedy that drawback, proposals are made in document GB 2 443 032 to modify the exciter to operate in a rotary transformer mode in order to deliver the excitation current for the rotor inducer of the main electrical machine on starting while it is operating in synchronous mode. That modification and the need to pass a high level of power via the stator of the exciter on starting at low speed mean that such a solution presents the drawback of extra cost in terms of weight and size.
Proposals have also been made to perform starting by causing the main electrical machine to operate in asynchronous motor mode rather than in synchronous motor mode. Reference may be made to documents U.S. Pat. Nos. 5,055,700, 6,844,707, and EP 2 025 926. In document U.S. Pat. No. 5,055,700, on starting, the stator windings of the main electrical machine are powered with AC via a starting contactor by means of an inverter circuit controlled with a constant ratio of voltage over frequency. The rotor of the main electrical machine is provided with damper bars that form a “squirrel cage” that enables the rotor to be set into rotation, while the rotor inducer of the main machine is periodically short-circuited by means of a special switch for avoiding damaging excess voltages. In document U.S. Pat. No. 6,844,707, on starting, stator windings of the main electrical machine are also powered with AC via a starting contactor by means of an inverter circuit that is controlled in voltage and in frequency. The rotor inducer of the main machine is short-circuited by a special switch that is initially closed. Short-circuiting the rotor inducer enables the rotor to be set into rotation in association with damper bars associated with the rotor inducer and forming a partial “squirrel cage”. The short-circuit switch is opened under the control of the current from the rotor windings of the exciter when the starter-generator changes to electricity generator mode. Document EP 2 025 926 also describes a main electrical machine operating in asynchronous motor mode on starting, the starting torque being provided by putting the rotor inducer in a closed circuit in series with a resistor by means of a switch, possibly with assistance from damper bars.
Since operation in asynchronous mode is degraded compared with operation in synchronous mode, those prior art solutions are not suitable for an S/G associated with turbine engines that require high power on starting, in particular with the turbine engines of propulsion aeroengines.
Furthermore, those prior art solutions require a controllable switch to be inserted in parallel or in series with the rotor inducer of the main electrical machine, and such an element is very likely to lead to unreliability.
Furthermore, it has been known for a long time to use asynchronous mode to start synchronous electric motors that are fitted with wound inducers or with bars that form a squirrel cage. The stage of starting until the synchronous speed is reached takes place solely in asynchronous mode. Reference may be made to documents U.S. Pat. No. 3,354,368 and GB 175084.